[Bug preprocessor/55715] New: bogus overflow warning for #if A-B when A<0 & B==minimum integer

[Bug preprocessor/55715] New: bogus overflow warning for #if A-B when A<0 & B==minimum integer

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55715

             Bug #: 55715
           Summary: bogus overflow warning for #if A-B when A<0 &
                    B==minimum integer
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: eggert@gnu.org


Consider this C program:

#if 9223372036854775807 < 0
#endif
#if -1 - 9223372036854775807 < 0
#endif
#if -1 - (-1 - 9223372036854775807) < 0
#endif

When compiled with GCC (x86-64), GCC 4.7.2 warns:

t.c:5:37: warning: integer overflow in preprocessor expression [enabled by
default]

The warning is incorrect, since there is no overflow in this program.

Evidently GCC's preprocessor computes A-B by first negating B, and then adding
the result to A.  This causes GCC to incorrectly report overflow when B is the
minimum possible integer and A is negative, because in this case -B overflows
even though A-B does not overflow.

[Bug preprocessor/55715] bogus overflow warning for #if A-B when A<0 & B==minimum integer

[jsm28 at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55715

--- Comment #1 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
Author: jsm28
Date: Tue Dec 10 01:23:37 2013
New Revision: 205846

URL: http://gcc.gnu.org/viewcvs?rev=205846&root=gcc&view=rev
Log:
    PR preprocessor/55715
libcpp:
    * expr.c (num_binary_op): Implement subtraction directly rather
    than with negation and falling through into addition case.

gcc/testsuite:
    * gcc.dg/cpp/expr-overflow-1.c: New test.

Added:
    trunk/gcc/testsuite/gcc.dg/cpp/expr-overflow-1.c
Modified:
    trunk/gcc/testsuite/ChangeLog
    trunk/libcpp/ChangeLog
    trunk/libcpp/expr.c

[Bug preprocessor/55715] bogus overflow warning for #if A-B when A<0 & B==minimum integer

[jsm28 at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55715

Joseph S. Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |4.9.0

--- Comment #2 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
Fixed for 4.9.